bind sometimes connection timed out

My dns is working perfectly fine for the past 8 yrs. Suddenly we are encountering connection timed out sometimes. I created a script to check the ports every 5 seconds and it is always open. My guess is it is the bind dns service that is being full. Is bind has some sort of limit tcp capacity? I notice this logs. It appears sometimes.

Quote:

TCP: Possible SYN flooding on port 53. Sending cookies.

Out of 20 nslookup, 2 of it is connection timed out. It seems it is intermittent. Has anyone here encountered the same issue?